Catadioptric lenses are used to provide extremely long focal lengths while requiring only a relatively short physical length of the lens. Primary and secondary reflective surfaces of mirrors within the lens are utilized to increase the length of the optical path through the lens while decreasing the overall length of the lens and thereby provide a very high telephoto ratio. The telephoto ratio is the ratio of the front vertex distance (FVD) of the lens to its equivalent focal length, the front vertex distance being the dimension from the front vertex of the lens to the film plane.
In catadioptric lenses of the type of which this invention pertains, light entering the housing passes to a primary mirror where it is reflected to a secondary mirror on the object side of the primary mirror, and hence to the refractive portion of the optical system. Most, if not all, catadioptric lenses are focused by relative motion of the primary and secondary mirrors. Optically, this is satisfactory. However, if the lens is to have a large relative aperture, large primary mirrors are required. These mirrors are inherently sensitive in positioning. Movement of the primary mirror requires large, and hence more expensive helicoidal focusing mechanisms.
It would be desirable to seal the main mirror system and achieve focusing by moving smaller refractive elements within the lens. However, this presents a difficult optical problem. Catadioptric systems covering the relatively large angles required for consumer photography, typically four to ten degrees full field angle, necessarily need rear refractive corrective elements to compensate for the inherent field curvature of the main mirror system.